In recent years, research and experiments have been made on quantum cryptography communication utilizing principles of quantum mechanics (Non Patent Documents 1 and 2). In the conventional quantum cryptography communication, studies have been made assuming that a photon in superposed states in the quantum mechanics exists in a communication path. When the photon in the superposed states is observed by an eavesdropper, the photon transitions from the superposed states to an eigenstate having definite information. Due to such an effect, the eavesdropper cannot perform eavesdropping without affecting the exchanged information, because the eavesdropper cannot reproduce the original superposed states. Hence, the fact that the proper sender and recipient can detect eavesdropping guarantees security.
For example, Non Patent Document 3 discloses an optical communication method for a quantum cryptography communication. In the optical communication method described in Non Patent Document 3, a sender phase-modulates a photon according to information desired to be transmitted and transmits the photon to a recipient. If an eavesdropper exists in a transmission pass and measures the photon, the eavesdropper may fail to retransmit a photon modulated by the same phase modulation because the eavesdropper cannot know the phase used by the sender. As a result, mismatch (error) between the information transmitted by the sender and the information received by the recipient increases and the existence of the eavesdropper can be thereby detected.